Childthemes for childthemes

I have watched the child theme videos and can only assume that Directory Theme has been updated since they were recorded, and I’m guessing that wordpress has also changed the directory structure of WP since the videos have been made, so I’m finding them hard to follow.

Currently, if I install one of the monthly PremiumPress child themes they are now stored in the wp-content theme directory with the prefix template_

(not as the video says, wp-content >> themes >> directorypress (now DT) >> themes >> directorypress-XXX (where XXX is the name of the child theme.)

Question. If I edit one of the template_xxx child themes will it be overwritten in some future update, or do I need to create a unique child theme (copied/based on a child theme)?